Over the last few years, speed limits on main thoroughways have been significantly reduced primarily as a result of fuel shortages. However, conventional engine construction permits the driving of vehicles at speeds well in excess of those called for by law, dictating the need for speed control devices such as governors on the vehicle. These are especially useful from both the fuel economy and safety standpoint on heavy-duty vehicles which are generally diesel powered.
The speed of the vehicle may be controlled by means of a device which limits or cuts off the fuel supply to the engine when a maximum preset speed is exceeded. For example, a fuel bypass line may be used in a diesel engine having a common rail line for supplying fuel to the engine from an injection pump. The fuel bypass line is closed by a valve at acceptable speeds and is only open at unacceptably high speeds. When the bypass line is open the fuel flows back to the fuel tank or into the inlet line, to the injection pump and the pressure in the common rail line is reduced so that less fuel flows to the engine.
However, the adjustment of the valve opening and closing the bypass lines presents certain operational problems. On the one hand, it is desirable to achieve maximum deceleration in which case, the dump valve to the bypass must be open to its maximum, however, the use of an uncontrolled wide open bypass has a potentially dangerous effect when the engine is idling. If the driver has exceeded the speed limit at which the uncontrolled dump valve is wide open with the accelerater pedal in the idling position as it would be for instance, when shifting gears or when declutching and disengaging the gears in an effort to increase speeds downhill, in neutral, virtually all of the fuel flows through the bypass thereby cutting off fuel flow through the common rail line to the extent that the engine stalls. Once the engine has stalled, it cannot be restarted above the preset speed limit simply because there is not enough fuel available through the rail line to the engine, nor can the gears be reengaged. Furthermore, a stalled engine deprives the vehicle of other essential auxilary power required to operate units such as the power steering and the air compressor, for the air brake system. For these, as well as other obvious reasons trying to restart the engine is a dangerous distraction for the driver when travelling at high speeds.
To overcome this difficulty, the dump valve to the bypass may be set up such that only a limited amount of fuel is dumped back to the return line to the tank or the fuel pump. This however, limits the reduction of fuel pressure in the fuel line itself, thereby taking away from the effectiveness of the bypass. For instance, when the fuel pump is set at high pressure operation required for full load and there is only limited reduction in fuel line pressure, the operator still has considerable power available from the engine and under certain conditions such as travelling with tail winds or travelling down grades, the driver is able to maintain speeds well in excess of the preset maximum.
The speed control system of the present invention provides an answer to the problems raised above, while at the same time, ensuring maximum deceleration at unacceptable high speeds. The system of the present invention comprises fuel line means to the vehicle engine, a fuel pump feeding the fuel line means, a fuel bypass which is normally closed by a speed control valve, from the fuel line means and means for monitoring the speed of the vehicle and for opening the fuel bypass when the vehicle speed reaches an unacceptable level, thereby bypassing fuel from the fuel line means and limiting fuel flow to the engine. The speed control valve in the fuel bypass opens wide at excessive speeds to provide the maximum deceleration, however, in order to avoid stalling the engine, the fuel bypass is provided with valve means adjusted to maintain the required amount of fuel flow through the fuel line means to the engine at all time for engine idling.